The invention relates to a cathode structure for a field emission device and method of fabricating the same.
One example of the field emission device includes a field emission display (FED) being a flat panel display. The field emission display comprises the base plate having a cathode and the face plate having phosphor, which are located in parallel positions separated by a short distance (less than 2 mm) vacuum-packaged. The field emission display is a device in which electrons emitted from the cathode in the base plate collide against a phosphor on the face plate to display image by means of a cathode luminescence of the phosphor. There has been a lot of study on a flat display that will replace a conventional cathode-ray tube (CRT).
The cathode, being one of main components of the FED, is very different in an electron emission efficiency depending on a device structure, an emitter material, the shape of an emitter, etc. At present, the structure of the field emission device is mainly classified into a diode-type structure consisting of a cathode electrode and an anode electrode, and a triode-type structure consisting of a cathode electrode, a gate electrode and an anode electrode. The emitter material may include metal, silicon, diamond, diamond-like carbon, carbon nanotube, etc. Generally, metal and silicon is used as emitter material in a cathode for a triode-type field emission device while diamond or carbon nanotube, etc. is used as emitter material in a cathode for diode-type field emission device. The diode-type field emission device mainly uses film or fiber, needle, particle or powder of diamond or carbon nanotube that has a good electron emission property in a low electric field, as the emitter material. The diode-type field emission device is disadvantageous in controllability of electron emission and a low-voltage driving, but it is advantageous in that it is simple in manufacturing process and has a high reliability of electron emission, compared to a triode-type field emission device.
Referring now to FIG. 1, there is shown a schematic view illustrating a conventional cathode structure for a diode-type field emission device disclosed in U.S. Pat. No. 5,900,301 issued to Brandes, etc.
A cathode 100 comprises a cathode electrode 140 on a base plate 120, a particulate emitter 160 on the cathode electrode 140, and a bonding material 170 for bonding the particulate emitter 160 to the cathode electrode 140. A glass substrate is usually used as a material of the base plate 120. The cathode electrode 140 can be fabricated by depositing metal on the glass substrate by means of sputtering process or electron beam process, etc. and then performing a selective etching process by means of photolithography process. A cathode electrode 140 usually uses metals having good electrical conductivity, which may include Cr, Ni, Nb, etc. An emitter 160 usually uses materials having a good electron emission characteristic at a low electric field, which may include materials containing carbon as the major ingredients such as diamond, diamond-like carbon, amorphous carbon, carbon nanotube, carbon nanoparticle, etc. It is preferred that the bonding material 170 uses an electrically conductive material having a high electrical conductivity since it must has the function of electrically connecting the emitter 160 to the cathode electrode 140. The bonding material 170 must also has the function of bonding the particulate emitter 160 to the cathode electrode 140.
The U.S. Pat. No. 5,900,301 describes that Ti, graphite, Ni or its alloy can be used as the bonding material 170, and also describes that a technology for increasing the bonding force between the emitter 160 and the cathode electrode 140. As another example, U.S. Pat. No. 5,948,465 issued to Blanchet-Fincher, etc. describes a metal compound as the bonding material 170 for bonding the emitter 160 to the cathode electrode 140. The two prior arts employ AgNO3 as the metal compound. One example for forming the bonding material 170 can be summarized as follows. A mixed solution is first prepared by adding 25 wt % AgNO3, 3 wt % polyvinyl alcohol (PVA), 71.9 wt % distilled water and a surface active agent of 0.1 wt % and is then coated on the cathode electrode to form a mixture film. Then, the particulate emitter material is uniformly distributed in the mixture film and then a heating step is performed. During the heating step, the mixture film is burnt, by which nonmetallic components constituting the mixed solution are thus removed to leave metal only. In case of using AgNO3 as the metal compound, Ag is left as the bonding material, which serves to not only electrically connect the emitter and the cathode electrode but also mechanically bond them.
FIG. 2 is a schematic view illustrating a conventional cathode 200 structure for a diode-type field emission device, disclosed in U.S. Pat. No. 5,623,180 issued to Jin, etc. The cathode 200 includes a cathode electrode 240 arranged in a stripe shape on a base plate 220, a particulate substrate 265 on the cathode electrode 240, and an emitter 260 covering the surface of the particulate substrate 265. It is mainly used an electrically insulator as material of the base plate 220. The cathode electrode 240 may be fabricated by using a good electrical conductor. A metal electrode having good electrical conductivity may be used as the material of the cathode electrode, and it is mainly used some materials having a good electron emission characteristic at a low electric field as the material of the emitter 260. Major materials of the emitter 260 may include diamond, ceramic particles such as oxide particles, nitride particles, carbon particle, etc. and semiconductor materials. As shown in FIG. 2, the emitter 260 bonded to the particulate substrate 265 may have a continuous phase that completely surrounds the particulate substrate 265. However, a plurality of the emitter particles may be discontinuously bonded to the particulate substrate 265. Some metal particles is usually used as the particulate substrate 265, and said metal may includes a metal that easily forms carbide such as Mo or a metal having high melting point. It is required that the size of the particulate substrate 265 be in the range of 0.1 to 100 micrometer in diameter, more preferably, in the range of 0.2 to 5 micrometer.
The method of fabricating the cathode electrode 240 in FIG. 2 is very different from that of fabricating the cathode electrode 140 in FIG. 1. The reason is that the cathode electrode 240 in FIG. 2 must serve to not only transfer an electrical signal to the emitter 260 but also bond the emitter 260 and the particulate substrate 265 to the cathode electrode 240. Therefore, the method of fabricating the cathode electrode 240 in FIG. 2 is similar to that of fabricating the bonding material 170 in FIG. 1. The method of fabricating the cathode electrode 240 can be summarized as follows. A slurry is produced by mixing a portion of liquid such as acetone, organic binder, metal or conductive oxide particles and particulate substrate 265 bonded with the emitters 260 by a given ratio. Metal particles may employ materials using Ag as the major ingredients and the conductive oxide particles may employ CuO particle that is easily reduced at low temperature. In a subsequent heating step, organic materials are burned out. After the heating step is finished, the particulate substrate 265 bonded to the emitter 260 and metal are left as residue. As shown in FIG. 2, the particulate substrate 265 surrounded by the emitters 260 after the heating step has a structure in which metal films are inserted discontinuously. The metal films function as the cathode electrode 240. In FIG. 2, as a portion of respective emitters 260 must have faceted edge so that it can be used as a field emission device, a surface treatment may be performed after the heating step in order to protrude the emitter 260. The surface treatment may include a chemical etching method, a mechanical polishing method, etc.
The diode-type cathodes used in the conventional field emission devices in FIGS. 1 and 2 have advantages in that the structure is simple and processes for manufacturing them are easy since they do not need a gate and a gate insulting film, unlike a conical triode-type cathode. Further, the cathode for a diode-type field emission device is high reliability because it is very robust in that the cathode is not easily broken by a sputtering effect upon emission of electrons. Also, there is rarely happened a breakdown on the gate and the gate insulating film, which becomes a big issue in the triode-type cathode. In addition, as shown in Korean Patent Application No. 99-31976, the need for a diode-type cathode as a new concept to development of an active-controlled diode-type field emission device becomes much greater.
The field emission device having the diode-type cathode has a structure in which a high electric field between the face plate and the base plate is necessary to emit electrons from the emitter. Thus, there is a limitation that the field emission device must use materials, which can easily emit electrons at a low electric field, as the material of the emitter. The materials of the emitter known so far include carbon containing materials such as diamond, diamond-like carbon, amorphous carbon, carbon nanotube, carbon nanoparticle, etc. Also, there has been reported that oxide, nitride, carbide, semiconductor materials can be used as the emitter material. However, any of them has not yet been implemented as a field emission device. The reason is that the emitter material having a good electron emission characteristic containing carbon nanotube is only synthesized at high-temperature process. Due to this reason, there are a lot of problems in selecting the base plate in order to form an emitter having a good electron emission characteristic.
In order to solve the above-mentioned problems, there was a need for a technology by which the particulate emitter material has been synthesized at a high-temperature process and the particulate emitter material is then bonded to the cathode electrode. As disclosed in several US patents (for example, U.S. Pat. No. 5,900,301, No. 5,948,465, No. 5,623,180), there is a great need of fabricating the diode-type cathode using the particulate emitter material. The key technology to be solved is the patterning of the particulate emitter material. In other words, there are a lot of problems in fabricating emitter suitable for a high-resolution field emission display device by means of conventional screen-printing method, spray coating method and dipping method.
In order to solve the above-mentioned problems in fabricating the cathode for the diode-type field emission device, the present invention proposes a method of fabricating a cathode for a field emission device using a photolithography process such as in FIG. 3. According to U.S. Pat. No. 5,064,396 issued to Spindt, the diode-type field emission device is disadvantageous in view of controllability of electron emission and low-voltage driving compared to the triode-type emission device. Another embodiment of the present invention proposes a method of fabricating a cathode for a field emission device using a lift-off process such as in FIG. 4. In a further embodiment of the present invention proposes a cathode structure for a triode-type field emission device capable of driving the field emission device at low voltage using a particulate emitter material, and a patterning process for using the particulate emitter material as a cathode.
A cathode for a field emission device proposed by the present invention has a base plate, a stripe-shaped metal electrode on the base plate, and an emitter of a particle shape or a powder shape that is bonded on the metal electrode by patterning. A glass plate, being an insulator, is used as the base plate. The cathode electrode is fabricated by forming an electrically conductive material by means of a physical vapor deposition method or a chemical vapor deposition method. It is appropriate that a metal is used as the material of the cathode electrode, and a material having a good electron emission characteristic at low electric field is used as the emitter. Representative emitter material may include materials containing carbon as the major ingredient, such as carbon nanotube, carbon nanoparticle, diamond having defects, ceramic particles such as oxide particles, nitride particles, carbon particles. Also, semiconductors are available.
A significant advantage of the present invention over the conventional art is that the present invention patterns an emitter material to a cathode electrode using photolithography process or a lift-off process. The present invention is characterized in that it forms an emitting compound in order to attach the emitter material to the cathode electrode. At this time, the emitting compound is a solution in which the emitter material is mixed with distilled water. Also, the emitting compound may include a binder for adjusting the viscosity and a small amount of additives. The viscosity and dispersion of the emitting compound can be controlled by means of the amount of the binder and additives. Also, the emitting compound is patterned using a lift-off process using a sacrifice layer after the compound film is uniformly formed on the base plate having the cathode electrode. In other words, after a sacrifice layer is formed on the cathode electrode, it is selectively exposed by ultra-violet light using a mask where is a desired pattern. Then, the sacrifice layer is selectively removed by means of a development process. Next, after the emitting compound is uniformly covered on the patterned sacrifice layer, as the emitting compound existing on the sacrifice layer is also removed by removing the sacrifice layer, patterning of the emitting compound can be thus obtained.
In the cathode for a field emission device, the emitter must exist at a desired portion. Therefore, the technology by which the particulate emitter material is bonded at a desired portion by patterning using a photolithography process disclosed in the present invention is significantly different from the conventional one. In the present invention, that is, the emitting compound formed on the cathode electrode can be exactly patterned at a desired portion since the sacrifice layer is patterned by photolithography process and the patterning of the sacrifice layer directly determines patterning of the emitting compound. As a result, the present invention can provide a technology necessary to fabricate an emitter for a high-resolution field emission device using emitter particles.
The method for fabricating a cathode for a field emission device proposed by the present invention is significantly different in the construction of the invention and its acting effect from the convention technologies. The particulate emitter is bonded to the cathode electrode by a lift-off process and patterning technology will be in detail explained by reference to FIG. 4.